The program you will need is called VirtualDub, you will want to follow that link and download the appropriate version (32 or 64bit).  Once you have downloaded and installed that program its just a few simple settings are were ready to go.

With the program open you’ll want to click on Video at the top and select “Direct Stream Copy” and in the Audio section we also want to select “Direct Stream Copy”.  With those settings in place your ready to start.

Take the 1st part of your .avi file and drop it into the program once there select “File” then “Append AVI Segement”.  This will place both parts in order.  Next you will simply press F7, choose where you would like the new file to be saved, rename it and click ok.

This process goes fairly quickly, for most 90-120min fils expect to spend 3-6min while it merges the two files together.  Once done you will have a nice new file of the entire movie that will be seamless.

Enjoy.